1. Field of the Invention
The present invention relates to a method for purification or refining crystal material, especially highly pure crystal raw material, for making crystals with less optical extrinsic absorption, especially by a multi-step treatment process for further purifying the raw material. It also relates to an apparatus for performing this method, which has a closable melt vessel with at least one closable opening and also to the use of the crystals obtained with this method.
2. Related Art
Crystals, especially single crystals, are increasingly used for making optical elements, such as lenses, etc. Many such crystals are used for high energy, especially short wavelength, radiation transmission and thus are suitable for making optical systems, such as lasers. They are also used in microlithography for manufacture of circuitry in small electrical components, such as computer chips. Not only ever larger sizes or geometries of lenses and/or single crystals are required, but also purer, especially optically purer, crystals are increasingly required because of the decreasing size of this sort of circuitry due to further miniaturization. Thus for example, crystal dislocations and especially schlieren formation or other defocusing structures in the photolithographic imaging circuitry, which can lead to short circuits and thus malfunction of the entire computer chip with increasing miniaturization, enormously increase the losses during chip production. Also the heat generated by absorption at impurity and defect sites leads to thermal expansion of the optical elements during photolithography and thus to focal point changes, which similarly lead to poorer image formation.
Methods for large-scale production of oriented single crystals already exist. All these methods have in common that a crystal seed is brought into contact with a melt of crystal raw material and the melt is cooled starting from the crystal seed, which determines the orientation of the crystal that is formed. Thus, for example, U.S. Pat. No. 2,149,076 describes a so-called vertical Bridgeman method, in which a melt vessel is surrounded by annular heating jackets arranged above each other. The melt vessel is moved and/or lowered along a central axis within these heating jackets. If the temperatures in the heating jackets arranged above each other are set so that the temperature in the upper jacket is above the melting temperature and the temperature in the lower jacket is below the melting temperature and the melt vessel is slowly lowered from the upper heating jacket into the lower heating jacket, the melt in the melt vessel solidifies starting to grow from a crystal seed, thus forming a single crystal.
In other methods, in which vertical crystal growth takes place, several heating elements that are fixed in relation to the melt vessel are provided, whose individual temperature are controlled during crystal growth. The so-called vertical gradient freeze (VGF) method is among these other methods. The heating elements are individually controlled so that a vertical temperature gradient is produced in the melt. In contrast to the Bridgeman method however the crystal growth is not obtained by lowering of the melt vessel, but by reduction of the power supplied to the heating elements, so that the melt is cooled starting from the vessel bottom and/or a crystal seed pocket generally in an upward direction and the phase boundary surface between melt and solid rises as the crystal grows. This sort of crystal growth method for, among others, magnesium fluoride, barium fluoride, strontium fluoride and calcium fluoride, is described, for example, in WO-A 01/649 75.
In order to attain the required purity of the crystals, they are preferably made from a synthetically manufactured powder of crystal material. Usually the bulk density of this material amounts to only about a third of that of the finished grown crystal, which means that a sufficient amount of the powder must be fed into the melt or growth vessel, which is at least three times the height of the finished crystal. Since the melting, purifying and growing of the crystal especially requires a time interval of usually several months, especially in the case of large size single crystals, the drying, purifying and melting of the crystal raw powder should be attempted in a separate simply structured melting apparatus. In this way the expensive highly sensitive crystal growing apparatus is not burdened by preparative process steps and can be used exclusively for the actual crystal growth method, which increases productivity.
The manufacture of individual crystal blanks or ingots in a preparation process can take place separately in a simple and economical melting apparatus, in which no direct crystal growth takes place. Instead a polycrystalline body, a so-called ingot, is produced. In this preparation process the conventional powdery raw material is melted and condenses to form a compact body and at the same time the material is purified in a vessel, which preferably has dimensions which are comparable to those of the later used growth vessel. In contrast to a crystal growth process, the material is quickly cooled to room temperature at the end of this melting process, since no crystal solidification is required and a polycrystalline body is obtained. This sort of polycrystalline body is usually designated an ingot. This polycrystalline ingot is then used as a blank or starting body for the actual crystal growth process. The separate formation of an ingot and the later crystal growth process is designated the two-step or two-stage process. A complete manufacturing method, in which melting the power, purifying it, homogenizing it, growing the crystal, tempering it and cooling it, are performed is designated a one-step or one-stage process.
It has been shown that the quality of the latter formed crystal depends very strongly on the process parameters for the drying, scavenging and homogenizing stages and indeed independently of whether these stages occur in a one-stage or two-stage ingot process. Thus, for example, DE-A 102 08 157 describes a method for making a fluoride crystal, in which a fluoride raw material is dried in an open vessel under vacuum. After closing the vessel the dried raw material is reacted with a scavenger called a de-oxidation means. After finishing the de-oxidation reaction the vessel cover is again opened in order the remove the scavenger reaction products and if necessary to finish melting the still not completely melted crystal raw material.